Various methods have been known as image-forming methods each of which is based on an electrophotographic method. A general image-forming method is as described below. A photoconductive substance is utilized so that an electrostatic latent image is formed on an electrostatic image bearing member (which may hereinafter be referred to as “photosensitive member”) with various means. Next, the electrostatic latent image is developed with toner to be turned into a visible image, and the visible image formed with the toner is transferred onto a recording medium such as paper as required. After that, the visible image is fixed as a toner image onto the recording medium with heat or pressure, whereby a copied article is obtained. An image-forming apparatus for use in such method is, for example, a printer or a copying machine.
In recent years, LED laser beam printers have gone mainstream in the market of printer apparatuses, and there has been a trend toward an increase in resolution: although conventional printer apparatuses each have a resolution of at most, for example, 300 dpi or 400 dpi, the LED laser beam printers each have a resolution as high as, for example, 600 dpi or 1,200 dpi. In association with the increase in resolution, definition requested of a developing system has been growing. In addition, as in the case of a printer, a high-resolution, high-definition developing system has been requested also of a copying machine because the functions of the copying machine have become more and more sophisticated by virtue of digitization.
In ordinary cases, toner to be used in such printer or copying machine is a mixture of fine particles each mainly composed of a binder resin and a colorant such as a dye, a pigment, carbon black, or a magnetic substance, and fine particles to be used in the toner each have a particle diameter of about 5 to 30 μm.
The toner is generally produced by the so-called pulverization method involving: melting the above colorant, and, as required, a charge control agent, a wax, and the like; mixing the molten product in a thermoplastic resin as the binder resin to disperse the molten product uniformly in the resin; finely pulverizing the resin composition thus obtained; and classifying the finely pulverized products to provide particles each having a desired particle diameter. A requirement that the above components should satisfy in the toner production is, for example, as follows: the above resin composition must be so sufficiently brittle as to be finely pulverized with an economical production apparatus. However, an increase in brittleness of the resin composition involves the following problem: the particle diameters of the particles obtained by finely pulverizing the resin composition are apt to cover a wide range. In addition, even after the particles have been turned into toner, the particles are apt to be additionally reduced in size during the use of the toner in a developing device, so the following problem also arises: a reduction in developing performance of the toner is caused by the exposure of the colorant to the broken-out section of a toner particle.
Meanwhile, a method of producing polymerized toner based on a suspension polymerization method has been proposed with a view to overcoming such problems of the toner by the pulverization method. The suspension polymerization method involves: dissolving or dispersing, in a polymerizable monomer, a colorant, and, as required, any other substance that needs to be incorporated into a toner particle such as a polyfunctional monomer, a chain transfer agent, a charge control agent, or a wax to prepare a polymerizable monomer composition; suspending, in an aqueous medium containing a dispersion stabilizer, the polymerizable monomer composition together with a polymerization initiator; and polymerizing the polymerizable monomer composition by a method such as heating to provide toner particles each having a desired particle diameter. Since the method does not involve any pulverizing step, a resin material does not need to have brittleness, and even a soft resin material can be used. In addition, the colorant is hardly exposed to the surface of a toner particle, so toner particles each of which: has uniform triboelectric chargeability; and is excellent in durability can be obtained. Further, a classifying step can be omitted, so a reducing effect on a cost for the production of the toner particles is improved because of, for example, energy savings, the shortening of a time period required for the production of the toner particles, and an increase in yield in which the toner particles are produced.
However, carbon black, and some dyes and pigments each of which is used as the above colorant are apt to inhibit a polymerization reaction. In addition, in polymerized toner produced by the suspension polymerization method or a resin produced by the suspension polymerization method, an unreacted polymerizable monomer may remain in a toner particle or resin particle depending on the kind of the polymerization initiator to be used. When the amount of the remaining polymerizable monomer becomes excessively large, the charge quantities of the individual toner particles become nonuniform, so fogging is apt to occur. In addition, the contamination of a toner carrying member or filming to a photosensitive member is apt to occur, so the following problem arises: a reduction in quality of an image formed with the toner occurs.
In addition, the efficiency with which the polymerization initiator is utilized in the suspension polymerization method is not always sufficient, and part of the molecules of the polymerization initiator may remain as a decomposition product residue in a toner particle or resin without being involved in a polymerization reaction. The decomposition product residue is produced as a result of, for example, the following behavior: free radicals (radicals) produced by the decomposition of the polymerization initiator each abstract a hydrogen atom from any other compound in a reaction system, or the radicals are disproportionated or recombine with each other. The decomposition product residue is mainly a compound such as an alcohol, a carboxylic acid, or a hydrocarbon. Of those decomposition products, a decomposition product having a low boiling point can be removed by distillation by performing an operation such as heating or decompression after polymerization, and a decomposition product having water-solubility can be eluted in the aqueous medium. However, it becomes difficult to remove a compound which: has a relatively high molecular weight; has a high boiling point; and is hardly soluble in water, and the compound remains in a toner particle.
Such decomposition product residue is also responsible for a reduction in charging stability of toner and a reduction in quality of an image formed with the toner during the long-term use of the toner. In addition, at the time of fixation, molten toner is apt to adhere to a heat roller, and the adhesion is one cause for the so-called hot offset in which a sheet to which an image is fixed is contaminated. In addition, a reduction in efficiency with which the polymerization initiator is utilized due to the production of a large amount of such decomposition product causes an increase in amount of an unreacted polymerizable monomer.
Investigation on a method of preventing an unreacted polymerizable monomer or a decomposition product residue derived from a polymerization initiator from remaining in a toner particle has been vigorously conducted so far, and such various methods as exemplified below have been proposed.
For example, the following method has been proposed (see Patent Document 1): a resin for toner in which the amount of the decomposition-product residue of a polymerization initiator is reduced is produced by using a peroxide having a specific structure and having a 10-hour half life temperature of 120° C. or lower as the polymerization initiator.
In addition, the following method has been proposed (see Patent Document 2): a resin for toner in which the remaining of an unreacted monomer (polymerizable monomer) is suppressed is obtained by performing polymerization in the coexistence of a polymerization initiator having another specific structure different from that of the above polymerization initiator and having a 10-hour half life temperature of 70° C. or higher, and any other polymerization initiator.
Further, the following method has been proposed (see Patent Document 3): a polymerized toner in which, for example, the amount of the decomposition product of a polymerization initiator or the amount of a remaining monomer (polymerizable monomer) is suppressed is produced by performing suspension polymerization using a non-aromatic organic peroxide having a molecular weight of 250 or less and a 10-hour half life temperature of 60 to 85° C. as a polymerization initiator in the polymerization temperature range of 75 to 100° C. in the production of a polymerized toner for a non-magnetic, one-component developer.
Of the above-mentioned conventional techniques, the method disclosed in Patent Document 1 involves the use of an aliphatic organic peroxide as a polymerization initiator, and an organic peroxide the number of carbon atoms of especially an aliphatic hydrocarbon group of which is limited out of, for example, ordinary peroxycarbonate organic peroxides, monocarbonate organic peroxides, diacyl organic peroxides, and dicarbonate organic peroxides is included in the category of such aliphatic organic peroxide. According to the method, a decomposition product derived from the polymerization initiator has a relatively low molecular weight. Therefore, when a binder resin for toner is produced by a solution polymerization method using the polymerization initiator, a decomposition product residue volatilizes by being heated at a high temperature in a solvent-removing step after polymerization or a melt-kneading step at the time of the preparation of toner, so the remaining of the decomposition product residue in a toner particle can be suppressed. However, when such polymerization initiator is applied to the production of toner by a suspension polymerization method, it is difficult to suppress the remaining of a decomposition product residue in a toner particle because the method does not involve any such step of heating the decomposition product residue at a high temperature as described above. In addition, it is also difficult to suppress the inhibition of polymerization by part of the molecules of a colorant.
In addition, the method disclosed in Patent Document 2 described above involves the use of a polymerization initiator that produces a radical which hardly causes a hydrogen abstraction reaction in the step of producing a binder resin for toner. According to the method, the radical is allowed to be present stably over a long time period, so the efficiency with which a monomer is utilized is improved, and the remaining of an unreacted monomer can be suppressed. However, the polymerization initiator is not always suitable as a polymerization initiator to be used in the production of toner by a suspension polymerization method because of its high 10-hour half life temperature. In addition, it is not true that only a radical which hardly causes a hydrogen abstraction reaction is produced from the polymerization initiator, and any other polymerization initiator must be further used in combination with the above polymerization initiator, so the method disclosed in Patent Document 2 is found to have a small reducing effect on the amount of a decomposition product residue to be produced.
Further, the method disclosed in Patent Document 3 described above specifies the molecular weight and 10-hour half life temperature of a polymerization initiator to be used in the production of polymerized toner by a suspension polymerization method, and intends to suppress the remaining of a decomposition product residue or of an unreacted monomer by means of the specification. However, the physical properties of a decomposition product are not uniquely determined merely by the molecular weight of the polymerization initiator, and are dominated by the molecular weight and molecular structure of the decomposition product itself. In addition, the amount of the unreacted monomer is not determined merely by the 10-hour half life temperature of the polymerization initiator, and depends largely on a balance between the 10-hour half life temperature and a polymerization temperature. In addition, the method intends to suppress the remaining of the decomposition product residue in a toner particle, not to suppress the very production of the decomposition product. According to the investigation conducted by the inventors of the present invention, the method is still susceptible to improvement in terms of the remaining of the decomposition product residue or of the unreacted monomer.
As described above, at present, no production method with which various deficiencies caused by the remaining of an unreacted polymerizable monomer or a decomposition product residue in a toner particle in polymerized toner by a suspension polymerization method can be solved has been obtained yet.
Patent Document 1: JP 61-114245 A
Patent Document 2: JP 07-181731 A
Patent Document 3: JP 3336862 A